I have a 1200GS (not A), and a F650GS (the 800cc twin)...so a little similar...
The 800 is much more nimble, and a delight on slower roads (i.e. small lanes in the Yorkshire dales), and light and flickable.
But... 2-up and luggage and it's the big brother every time....
The 800 just can't compete in the lower end of the rev scale, at normal A-road speeds, the 800 lacks the type of easy 50-60mph overtaking the 1200 just laps-up. The 800 seems to run out of straight road before you've finished overtaking. Lots more gearbox stirring needed to ride briskly (when the 1200 stays in top gear and rolls on/off the throttle).
Personally I like them both (a lot!), it depends on the riding you plan to do, and whether you usually carry a passenger.
Last year I rode from Amsterdam to Calais, then Dover to home on the 1200 (two-up with luggage). It just breezed along effortlessly. On the 800 it would have been less comfy, and more of a chore.
Round the Dales, the 800's a delight (no pillion).
Planning long rides on both this summer, but 2-up (in a hurry!) it will always be the 1200...